The researchers of the first project explained that in theory, graphene should represent an ideal ultrathin barrier layer, as the pores between carbon atoms are smaller even than the radius of a helium atom. In practice, however, crystal boundaries and missing atoms allow vapor to permeate through the material, and the weak van der Waals bonds between planes mean that even stacks of multiple graphene layers can be penetrated. The solution reported by the team is to take a graphene monolayer formed by CVD, and to then use atomic layer deposition (ALD) to coat it with a 25–50 nm thick layer of alumina. Achieving conformal coatings on single-layer graphene is known to be difficult due to the material’s strong hydrophobicity.
The researchers found, however, that additional seed layers or pre-functionalizing of the graphene are not needed if the coating is applied immediately after the CVD stage, while the graphene–substrate complex is still hydrophilic, or if prolonged residence times are used to achieve optimal saturation conditions. The resulting nanoscale composite is suitable for metal passivation, device encapsulation and transparent barrier films.
Although a single layer of graphene paired with 50 nm of alumina does not achieve the extreme impermeability required for high-sensitivity applications like OLED encapsulation, the CVD–ALD process can be repeated until the necessary water vapour transmission rate (WVTR) has been reached. Barrier layers fabricated using this technique could exhibit appropriately low transmission rates with thicknesses of just tens of nanometres, compared to the millimetre-or-thicker layers used currently in televisions and smartphones.
Graphene is also attractive for microelectronics applications. Here, too, ALD alumina can be applied to coat and separate layers of active graphene in multilevel stacked devices. This means that delicate graphene structures can be protected during fabrication and processing, and their properties kept stable over time.
The researchers from Imperial College London reported the deposition of a metal-oxide layer on graphene, but in this case the material was strontium titanate (SrTiO3), with aims of fabricating a tuneable capacitor. The research reveals for the first time the growth mechanism of epitaxial oxide thin films on graphene transferred onto SrTiO3 and MgO substrates.